The disclosure relates to blade clearance in turbomachinery. More particularly, the disclosure relates to control via thermal properties of shroud support rings.
Gas turbine engines may contain rotating blade stages in fan, compressor, and/or turbine sections of the engine. Clearance between blade tips and the adjacent non-rotating structure may influence engine performance. Clearance may be influenced by mechanical loading (e.g., radial expansion of the blades and/or their supporting disks due to speed-dependent centrifugal loading) and thermal expansion (e.g., of the blades/disks on the one hand and the non-rotating structure on the other).
The high temperatures of the turbine section(s) make clearance issues particularly significant due both to: (1) the greater significance of thermal expansion; and (2) temperature-induced modulus reduction which exacerbates expansion from mechanical loading. In multi-spool engines, this will be particularly significant in the high speed/pressure turbine section of the engine. This may be particularly significant in the engines of combat aircraft which may be subject to greater and more rapid variations in speed and other operating conditions than are the engines of civil aircraft.
Accordingly, a variety of clearance control systems have been proposed.
To provide active control, many proposed systems form the non-rotating structure with a circumferential array of blade outer air seal (BOAS) segments mounted for controlled radial movement (e.g., via actuators such as electric motors or pneumatic actuators). An aircraft or engine control system may control the movement to maintain a desired clearance between the inner diameter (ID) faces of the BOAS segments and the blade tips.
Additionally, various proposed systems have involved tailoring the physical geometry and material properties of the BOAS support structure to tailor the thermal expansion of the support structure to provide a desired clearance when conditions change. Such thermal systems may be passive. Alternatively, such thermal systems may involve an element of active control such as via controlled direction of cooling air to the support structure.
Proposals for thermal expansion-based systems have included systems wherein the BOAS support structure comprises two distinct materials having different coefficients of thermal expansion (CTE) and dimensioned and positioned relative to each other to provide a staged expansion wherein the relative influence of each of the two materials changes over the range of operation. One example of such a system is found in U.S. Pat. No. 5,092,737.